1. Field of the Invention
This invention relates to an antenna apparatus used for satellite communication and, specifically, to an antenna apparatus used at an earth station provided with a radio telescope.
2. Description of the Prior Art
FIG. 15(a) shows the configuration of a conventional offset paraboloidal reflector antenna apparatus described, for example, in "Very Small Aperture Terminal for Satellite Communication", Mitsubishi Electric Technical Report, vol. 71, No. 9, 1997. Reference numeral 1 denotes an offset paraboloidal reflector antenna (to be referred to as "paraboloidal reflector antenna" hereinafter) which is a main reflector, 2 a horn having a circular aperture for both transmission and reception, 3 a feeder circuit, 4 a transmitter, and 5 a receiver. Reference letter P indicates the focusing point of the paraboloidal reflector antenna 1.
In satellite communication, different frequencies are allocated to transmission and reception. For example, when the frequency band is a Ku band, 14 GHz is used for transmission from the earth to the satellite and 12 GHz is used for transmission from the satellite to the earth (reception on the earth). In this case, the feeder circuit 3 comprising a transmit-receive branching filter 30A is connected to the horn 2 as shown in FIG. 15(b) and the transmitter 4 and the receiver 5 are connected to the above transmit-receive branching filter 30A. When the frequency band is a Ka band, 30 GHz is used for transmission from the earth to the satellite and 20 GHz is used for reception on the earth. In this case, as shown in FIG. 15(c), the feeder circuit 3 comprises a circular polarizer 30B and the transmit-receive branching filter 30A. In either case, the horn 2 and the transmit-receive branching filter 30A or the circular polarizer 30B must be designed to be used for both transmission and reception. The configuration of the feeder circuit 3 shown in FIG. 15(b) is used in a satellite system using linearly polarized waves and the configuration of the feeder circuit 3 shown in FIG. 15(c) is used in a satellite system using circularly polarized waves.
A description is subsequently given of the operation of the above-structured antenna apparatus. In the satellite system using linearly polarized waves shown in FIG. 15(b), waves from the satellite are received by the paraboloidal reflector antenna 1 and guided from the horn 2 to the receiver 5 through the transmit-receive branching filter 30A. On the other hand, signals from the transmitter 4 are transmitted to the horn 2 through the transmit-receive branching filter 30A and radiated from the paraboloidal reflector antenna 1 to the satellite. In the satellite system using circularly polarized waves shown in FIG. 15(c), waves (circularly polarized waves) received from the satellite are transmitted from the horn 2 to the circular polarizer 30B to be converted into linearly polarized waves which are then guided to the receiver 5 through the transmit-receive branching filter 30A. Transmission waves (linearly polarized waves) from the transmitter 4 are transmitted from the transmit-receive branching filter 30A to the circular polarizer 30B to be converted into circularly polarized waves which are then transmitted to the horn 2 to be radiated from the paraboloidal reflector antenna 1 to the satellite.
In the conventional antenna apparatus, since the horn, the transmit-receive branching filter and the circular polarizer are used for both transmission and reception as described above, they must operate at a wide frequency band including a transmission frequency band and a reception frequency band. Therefore, they become bulky and are not economical. For instance, in the case of the above Ka band, a 20 GHz frequency band of 17.7 to 21.2 GHz (bandwidth of 3.5 GHz) is allocated to reception and a 30 GHz frequency band of 27.5 to 31.0 GHz (bandwidth of 3.5 GHz) is allocated to transmission in satellite communication. That is, the bandwidth ratio (percentage of the ratio of bandwidth to band average frequency) of each frequency band is 18% for a 20 GHz frequency band and 12% for a 30 GHZ frequency band. When the frequency band is used for both transmission and reception, the frequency band for both transmission and reception ranges from 17.7 to 31.0 GHz (bandwidth of 13.3 GHz) and hence, the bandwidth ratio is 55%. Since the use frequency band is broad when the frequency band is used for both transmission and reception, in the design of an antenna apparatus which uses a frequency band for both transmission and reception, circuit design becomes complicated, circuit scale becomes large, high work accuracy is required, and electrical adjustment takes time to achieve targeted performance. In addition, skill for the precise adjustment of a circuit is required. Thus, compared with the design of an antenna apparatus which uses different frequency bands exclusive for transmission and reception, this antenna apparatus involves a large number of problems to be solved.
To cope with the above problems, an antenna apparatus equipped with primary radiators, each comprising a horn and a feeder circuit for each frequency band, as shown in FIG. 16(a) is conceivable. This antenna apparatus comprises a horn 2A dedicated for a lower frequency (f.sub.1) and a horn 2B dedicated for a higher frequency (f.sub.2) which are arranged near the focusing point P of the paraboloidal reflector antenna 1. The horn 2A is connected to a feeder circuit 3A dedicated for f.sub.1 and a feeder circuit 3B dedicated for f.sub.2.
Since the horns 2A and 2B must be shifted away from the focusing point P of the paraboloidal reflector antenna 1 in a direction perpendicular to the axial direction, radiation patterns from the paraboloidal reflector antenna 1 are displaced from the front direction of the paraboloidal reflector antenna 1 as shown in FIG. 16(b). The radiation patterns of f.sub.1 and f.sub.2 are displaced in opposite directions. That is, the radiation patterns from the paraboloidal reflector antenna 1 cause the displacement of a beam which is determined by the off-axis shift of the horns and the parameter of the paraboloidal reflector antenna 1. When the paraboloidal reflector antenna 1 is directed toward the front direction, neither the radiation pattern of fi nor the radiation pattern of f.sub.2 does not take maximal values. Therefore, when the two horns for f.sub.1 and f.sub.2 are arranged near the focusing point of the paraboloidal reflector antenna 1, the best solution that the operation gains of both f.sub.1 and f.sub.2 become maximal cannot be obtained due to the displacement of a beam during actual operation. That is, when the paraboloidal reflector antenna 1 is directed toward the front direction, the operation gains of both f.sub.1 and f.sub.2 become lower than their maximum gains. When the paraboloidal reflector antenna 1 is directed toward a direction in which the operation gain of one frequency (for example, f.sub.1) becomes maximal, the operation gain of the other frequency (f.sub.2) lowers.
An antenna apparatus having a plurality of horns is disclosed, for example, by Japanese Laid-open Patent Application No. Sho 56-119504. FIG. 17(a) shows the configuration of the antenna apparatus and FIG. 17(b) is a front view of its horns. In the antenna apparatus in which a center horn 2Z is used and multiple frequencies are shared, since the focusing point P of the paraboloidal reflector antenna 1 is aligned with the center horn 2Z for transmission, the center horn 2Z determines the array interval between peripheral horns 2C and 2D for reception. Thus, the relationship among the center frequency and the peripheral frequency is limited. That is, the above-structured antenna apparatus is effective when the transmission frequency is separated from the reception frequency (about 5 times in the above example). However, since the center frequency (transmission) is 30 GHz and the peripheral frequency (reception) is 20 GHz in the case of the above-described Ka band, the difference between these frequencies is small (1.5 times), whereby the synthesized primary radiation pattern at 20 GHz becomes too narrow.
FIG. 18(a) shows the configuration of an antenna apparatus having a plurality of horns disclosed by Japanese Laid-open Patent Application No. Sho 55-153402 in which a pair of horns 2E and 2F are arranged around the focusing point P of the paraboloidal reflector antenna 1 in such a manner that the center axes of the horns cross each other at the focusing point P to enhance the performance of the antenna. That is, as shown in FIG. 18(b), the center axes of the horns 2E and 2F are inclined toward the symmetry plane Sq of the paraboloidal reflector antenna 1 so that the horns 2E and 2F can radiate waves onto left and right portions of a reflector, respectively. Owing to this, radiation patterns onto the reflector surface of the paraboloidal reflector antenna are controlled and the level of field strength synthesized by the horns 2E and 2F in a front direction is made flat as shown in FIG. 18(c) to achieve high performance for the antenna. When the phase center of each horn (focusing points of the horns denoted by 11E and 11F of FIG. 18(b)) is not located on the center axis of each horn and outside each horn, it is impossible to align the focusing point P of the paraboloidal reflector antenna 1 with the phase center of each horn. However, in the case of a rectangular horn having a square aperture, as the phase center is located on the aperture of the horn or within the horn, an antenna apparatus as described above cannot be constructed using the rectangular horn.
It is possible to bring the phase centers 12E and 12F of the horns 2E and 2F a little close to the focusing point P by inserting a dielectric bar 2G into the horns 2E and 2F in such a manner that it extends from the root of each horn through the aperture to the outside as shown in FIG. 18(d). However, it is structurally difficult to align the focusing point P of the paraboloidal reflector antenna 1 with the phase center of each horn.